The present invention relates to a MIG (Metal Inert Gas) process for the electric-arc welding of aluminum and aluminum alloys, either in a pulsed mode or in a spray mode without modulated current.
The MIG electric-arc welding process is widely used in industry, including that of welding aluminum.
Shielding gases play an essential role in the performance of this process.
Thus, argon does not allow high welding speeds and may give rise, in automatic welding, to arc instabilities above 500 A. The characteristic shape of the narrow penetrations obtained with argon in spray mode is ill-suited to welding by interpenetration.
Moreover, argon/helium (hereafter Ar/He) mixtures and helium by itself make it possible to increase the depth of penetration and its root width, and hence make it possible to dispense with expensive preparations, the more so the higher the helium content in the Ar/He mixtures.
In other words, for a constant thickness, helium therefore allows welding speeds which increase with its content.
However, although the compactness of the beads is also generally improved by the presence of helium, this is to the detriment of the appearance of the beads, which are less shiny than with pure argon.
Ar/He mixtures therefore present an appreciable advantage in terms of quality and productivity, both in manual welding (for example for a mixture of the Ar/20% He type) and in automatic welding (for example for mixtures of the Ar/50% to 70% He type) with, however, a not insignificant cost associated with the helium content.
For applications not necessarily having to meet these two criteria, it may be judicious to consider other types of mixtures.
Thus, document EP-A-639 423 proposes, for TIG and MIG processes, to use a welding gas of the argon or argon/helium type containing, furthermore, from 100 to 1000 ppm by volume of CO2 and/or O2.
Furthermore, document DE-A-4241982 proposes to use argon or an Ar/He mixture to which has also been added from 80 to 250 ppm by volume of nitrogen.
Document EP-A-442 475 recommends welding with a consumable electrode using a gas mixture consisting of 0.5 to 1.25 vol % carbon dioxide, 30 to 40 vol % helium and the balance being argon.
Document U.S. Pat. No. 4,071,732 relates to a mixture formed from an inert gas containing less than 30% carbon dioxide or less than 5% oxygen; however, the examples in this document describe only mixtures of argon and carbon dioxide with contents of about 5 or 15%.
It should be emphasized that in terms of increasing the performance, none of these processes is fully satisfactory from the industrial standpoint.
Furthermore, in modulated spray MIG welding, that is to say with the welding current being modulated, it has already been recommended to use a shielding gas or gas mixture formed from argon, helium or mixtures thereof, to which from 0.01% to 1.80% of carbon dioxide and/or oxygen have been added, as described in EP-A-909 604. However, in this case, current modulations at a frequency of less than 60 Hz are applied to the current so as to be able to degas the weld puddle in order to remove gaseous inclusions therefrom, particularly diffusable hydrogen, liable to be found therein. This is because MIG processes in spray mode with current modulations are used when it is desired to obtain a high quality of the welded joint, but without actually needing to achieve a high welding speed.
Consequently, the problem that arises is to improve the known MIG welding processes not using modulation of the welding current, particularly MIG processes in unmodulated spray mode, that is to say without modulation of the welding current, and processes in pulsed mode, so as to be able to obtain high performance levels in terms of productivity and welding speed.
This is because MIG processes in unmodulated spray mode (without modulation of the current) and those in pulsed mode are much more suitable when it is desired to improve productivity rather than quality, that is to say the appearance of the welds.
Hitherto, MIG processes in unmodulated mode or in pulsed mode are used little or not at all for welding aluminum or its alloys when the gas shield contains oxygen.
This is because it is usually recognized that the presence of oxygen in the gas shield may have a deleterious effect on the weld given that, when oxygen is incorporated into the gas shield, the oxygen can easily combine with aluminum atoms and result in solid inclusions of alumina (Al2O3) in the weld, which have a negative effect on the mechanical properties of said weld. Moreover, this has been confirmed for high oxygen contents, that is to say oxygen contents greater than 2%, and for high carbon dioxide contents, that is to say, again, contents greater than 2%.
However, conversely, the presence of oxygen in the shielding gas stream results in productivity performance levels which are acceptable from the industrial standpoint.
It therefore follows that the problem that arises is to provide an MIG welding process for aluminum and its alloys resulting both in a high and industrially acceptable productivity and a low level of alumina inclusions in the weld without a major or appreciable impact on the mechanical properties of the welded joints.
The solution provided by the present invention therefore relies on a process for MIG welding, in spray mode without current modulation or in pulsed mode, of aluminum and aluminum alloys, with the use of a gas shield for at least part of the welding zone, wherein the gas shield is a gas mixture consisting of from 0.01% to 1.80% oxygen and from 20% to 98.2% helium, any balance consisting of argon.
Further characteristics of the process of the invention are given below:
the shielding gas contains from 0.9% to 1.80% oxygen and from 15% to 98.20% helium, the balance being argon;
the shielding gas mixture contains at least 1% oxygen, preferably at least 1.1% oxygen, more preferably at least 1.2% oxygen;
the shielding gas mixture contains at most 1.70% oxygen, preferably at most 1.65% oxygen;
a solid meltable wire is used;
the welding speed is from 0.25 m/min. to 1.20 m/min., preferably from 0.60 to 1 m/min.;
the wire speed is from 2.5 m/min. to 20 m/min., preferably from 4 m/min. to 17 m/min.;
the mean welding current is from 40 A to 450 A and/or the mean welding voltage is from 15 V to 40 V;
the process is in pulsed mode and/or the welding current is from 120 A to 350 A and/or the mean welding voltage is from 20 V to 30 V;
the process is in spray mode and/or the welding current is from 180 A to 450 A and/or the mean welding voltage is from 20 V to 39 V.
The present invention therefore relies on precise control of the oxygen content in the helium or an argon/helium mixture, it being necessary for the maximum oxygen content not to exceed about 1.80%, the gas mixture thus formed constituting the gas shield used when implementing the MIG process.
It should be emphasized that any MIG process in spray mode with modulation of the welding current is excluded from the invention.